1. Field of the Invention
The present invention relates to a switching mode power supply and, more particularly, to a forward converter for improving a power factor and suppressing a harmonic noise component appearing at an input current waveform. The present invention is based on Korean Application Nos. 57210/1996 and 58602/1996, which are incorporated herein by reference.
2. Description of the Related Art
A switching mode power supply (hereinafter, referred to as the "SMPS") represents power supply equipment for converting an input voltage into a stable voltage according to an operation of a switching mode and supplying the stable voltage to a load. Since the SMPS has a small size, is light in weight and has high efficiency, it meets the demands of the times such as energy saving etc. The SMPS having such features can be utilized as a forward converter and a flyback converter depending on a transformer for transmitting energy to a secondary winding according to the state of a switching element connected to a primary winding.
FIG. 1 illustrates the construction of a conventional forward converter. A bridge diode BD, a capacitor C1, a switching element Q, a diode D1, a control circuit 2, a primary winding Lp of a transformer T1, and a reset winding Lr of the transformer T1 are connected to an input side. A secondary winding Ls of the transformer T1, diodes D2 and D3, a choke coil L1, a capacitor C2 and a load 4 are connected to an output side.
The bridge diode BD full-wave rectifies an AC (Alternating Current) input voltage Vinac. The capacitor C1 smooths the voltage full-wave rectified by the bridge diode BD and supplies the smoothed DC (Direct Current) voltage to the control circuit 2 and the primary winding Lp of the transformer T1. The control circuit 2 generates a switching control signal having a prescribed period in response to an input of the DC voltage smoothed by the smoothing capacitor C1. The switching element Q connected between the primary winding Lp of transformer L1 and a ground terminal is switched on or off in response to the switching control signal generated from the control circuit 2. The transformer T1 includes the primary winding Lp connected between the smoothing capacitor C1 and the switching element Q, the reset winding Lr connected between the smoothing capacitor C1 and the diode D1, and the secondary winding Ls connected to the load 4. If the switching element Q is switched on, the DC voltage smoothed by the smoothing capacitor C1 is accumulated in the primary winding Lp and induced to the secondary winding Ls. If the switching element Q is switched off, energy accumulated in the primary winding Lp is transmitted to the ground terminal through the reset winding Lr and the diode D1. Since the winding Lr and the diode D1 perform such a function, they are called a reset winding and a reset diode, respectively, and prevent the switching element Q from being damaged. The diodes D2 connected between one terminal of the secondary winding Ls of the transformer T1 and the load 4 through choke coil L1, and the diode D3 connected between the other terminal of the secondary winding Ls of the transformer T1 and the load 4 through choke coil L1 are rectifying elements for rectifying the voltage induced to the secondary winding Ls of the transformer T1. The smoothing capacitor C2 connected in parallel to the load 4 smooths the voltage generated through the diode D2 or D3 so that a stable voltage can be supplied to the load 4. The diode D3 is typically called a flywheel diode.
If the AC input voltage Vinac is applied, the bridge diode BD and the smoothing capacitor C1 rectify and smooth the AC input voltage Vinac and generate a corresponding DC voltage. This DC voltage is supplied to the control circuit 2 and the primary winding Lp of the transformer T1. The control circuit 2 is operated according to the DC voltage and generates the switching control signal. The switching element Q is repeatedly switched off and on in response to the switching control signal. The transformer T1 induces the DC voltage supplied to the primary winding Lp to the secondary winding Ls as the switching element Q is switched on or off. The DC voltage induced to the secondary winding Ls of the transformer T1 is rectified by the output diode D2 or the flywheel diode D3 and smoothed by the capacitor C2 via the choke coil L1. The smoothed DC voltage is supplied to the load 4.
As indicated in FIG. 1, the capacitor C1 for smoothing the voltage rectified by the bridge diode BD is connected to the bridge diode BD. Therefore, a charge current flowing into the capacitor C1 approximates to a pulse form as shown in FIG. 2D. That is, a waveform of the voltage (ripple voltage) rectified by the bridge diode BD and a waveform of the voltage smoothed by the capacitor C1 continue to be repeated with a half period between 0 and a maximum value. In this case, the voltage smoothed by the capacitor C1 has a longer high voltage interval than the voltage rectified by the bridge diode BD. During this interval, there is no current flowing into the capacitor C1, and only during a very short interval around the maximum value of the ripple voltage, a current flows into the capacitor C1. Therefore, the current flowing into the capacitor C1 approximates to the pulse form. This form of the current may deteriorate a power factor and generate harmonic noise, thereby leading to a malfunction in the forward converter and increasing a reactive power.